TLS/SSL Module

What is TLS/SSL?

Transport Layer Security (TLS) and its predecessor, Secure Socket Layer (SSL), are protocols that provide secure communication over a computer network. They ensure:

• Privacy: Communications are encrypted to prevent eavesdropping
• Data integrity: Message contents cannot be modified without detection
• Authentication: The identities of the communicating parties can be verified

TLS/SSL is commonly used for securing:

• Web browsing (HTTPS)
• Email transmissions (SMTP, IMAP, POP3)
• Instant messaging
• Voice over IP (VoIP)
• API communication

Using the TLS Module

To use the TLS module in Node.js, you need to require it:

const tls = require('tls');

TLS Server

Here's how to create a basic TLS server:

const tls = require('tls');
const fs = require('fs');
const path = require('path');

// Server options with TLS certificates
const options = {
  key: fs.readFileSync(path.join(__dirname, 'server-key.pem')),
  cert: fs.readFileSync(path.join(__dirname, 'server-cert.pem')),
  // Request client certificate (optional)
  requestCert: true,
  // Reject connections without authorized certificates (optional)
  rejectUnauthorized: false
};

// Create TLS server
const server = tls.createServer(options, (socket) => {
  console.log('Server connected',
    socket.authorized ? 'authorized' : 'unauthorized');

  // Set encoding for data
  socket.setEncoding('utf8');

  // Handle incoming data
  socket.on('data', (data) => {
    console.log('Received:', data);
    // Echo back the data
    socket.write(`You said: ${data}`);
  });

  // Handle socket closure
  socket.on('end', () => {
    console.log('Socket ended');
  });

  // Write welcome message
  socket.write('Welcome to the TLS server!\n');
});

// Start TLS server
const port = 8000;
server.listen(port, () => {
  console.log(`TLS server running on port ${port}`);
});

Note: This example requires certificate files (server-key.pem and server-cert.pem). For development purposes, you can generate self-signed certificates using OpenSSL.

Generating Self-Signed Certificates for Development

You can use OpenSSL to generate self-signed certificates for development and testing:

# Generate CA certificate
openssl genrsa -out ca-key.pem 2048
openssl req -new -x509 -key ca-key.pem -out ca-cert.pem -days 365

# Generate server certificate
openssl genrsa -out server-key.pem 2048
openssl req -new -key server-key.pem -out server-csr.pem
openssl x509 -req -in server-csr.pem -CA ca-cert.pem -CAkey ca-key.pem -CAcreateserial -out server-cert.pem -days 365

# Generate client certificate (optional, for mutual authentication)
openssl genrsa -out client-key.pem 2048
openssl req -new -key client-key.pem -out client-csr.pem
openssl x509 -req -in client-csr.pem -CA ca-cert.pem -CAkey ca-key.pem -CAcreateserial -out client-cert.pem -days 365

TLS Client

Creating a client that connects to a TLS server:

const tls = require('tls');
const fs = require('fs');
const path = require('path');

// Client options
const options = {
  // For mutual authentication (optional)
  key: fs.readFileSync(path.join(__dirname, 'client-key.pem')),
  cert: fs.readFileSync(path.join(__dirname, 'client-cert.pem')),
  // Server name for Server Name Indication (SNI)
  servername: 'localhost',
  // CA certificate to verify the server (optional)
  ca: fs.readFileSync(path.join(__dirname, 'ca-cert.pem')),
  // Reject unauthorized certificates
  rejectUnauthorized: true
};

// Connect to server
const client = tls.connect(8000, 'localhost', options, () => {
  // Check if authorized
  console.log('Client connected',
    client.authorized ? 'authorized' : 'unauthorized');

  if (!client.authorized) {
    console.log('Reason:', client.authorizationError);
  }

  // Send data to server
  client.write('Hello from TLS client!');
});

// Set encoding for received data
client.setEncoding('utf8');

// Handle received data
client.on('data', (data) => {
  console.log('Received from server:', data);

  // Send another message
  client.write('How are you?');
});

// Handle errors
client.on('error', (error) => {
  console.error('Connection error:', error);
});

// Handle connection end
client.on('end', () => {
  console.log('Server ended connection');
});

// Close connection after 5 seconds
setTimeout(() => {
  console.log('Closing connection');
  client.end();
}, 5000);

Server and Client Options

Both tls.createServer() and tls.connect() accept various options to configure the TLS connection:

Common Options

• key: Private key in PEM format
• cert: Certificate in PEM format
• ca: Trusted CA certificates
• ciphers: Cipher suite specification string
• minVersion: Minimum TLS version to allow
• maxVersion: Maximum TLS version to allow

Server-specific Options

• requestCert: Whether to request a certificate from clients
• rejectUnauthorized: Whether to reject clients with invalid certificates
• SNICallback: Function to handle SNI from the client

Client-specific Options

• servername: Server name for SNI
• checkServerIdentity: Function to verify server hostname
• session: A Buffer instance containing TLS session

const tls = require('tls');
const fs = require('fs');

// Comprehensive server options
const serverOptions = {
  // Key and certificate
  key: fs.readFileSync('server-key.pem'),
  cert: fs.readFileSync('server-cert.pem'),

  // Certificate Authority
  ca: [fs.readFileSync('ca-cert.pem')],

  // Protocol version control
  minVersion: 'TLSv1.2',
  maxVersion: 'TLSv1.3',

  // Cipher control
  ciphers: 'ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384',

  // Client authentication
  requestCert: true,
  rejectUnauthorized: true,

  // Server Name Indication handling
  SNICallback: (servername, cb) => {
    // Different certificates for different servernames
    if (servername === 'example.com') {
      cb(null, tls.createSecureContext({
        key: fs.readFileSync('example-key.pem'),
        cert: fs.readFileSync('example-cert.pem')
      }));
    } else {
      // Default certificate
      cb(null, tls.createSecureContext({
        key: fs.readFileSync('default-key.pem'),
        cert: fs.readFileSync('default-cert.pem')
      }));
    }
  }
};

// Example client options
const clientOptions = {
  key: fs.readFileSync('client-key.pem'),
  cert: fs.readFileSync('client-cert.pem'),
  ca: [fs.readFileSync('ca-cert.pem')],

  servername: 'example.com',
  minVersion: 'TLSv1.2',

  // Custom identity check function
  checkServerIdentity: (hostname, cert) => {
    // Custom validation logic
    if (hostname !== cert.subject.CN) {
      return new Error(`Certificate CN does not match hostname: ${hostname}`);
    }
    return undefined; // No error
  },

  // Session reuse
  session: savedTlsSession, // Previously saved session
};

Secure HTTP Server (HTTPS)

While the TLS module can be used directly, for HTTPS servers, Node.js provides a higher-level https module built on top of TLS:

const https = require('https');
const fs = require('fs');
const path = require('path');

// HTTPS server options
const options = {
  key: fs.readFileSync(path.join(__dirname, 'server-key.pem')),
  cert: fs.readFileSync(path.join(__dirname, 'server-cert.pem'))
};

// Create HTTPS server
https.createServer(options, (req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/html' });
  res.end('<h1>Secure HTTPS Server</h1><p>This connection is encrypted using TLS.</p>');
}).listen(443, () => {
  console.log('HTTPS server running on port 443');
});

The HTTPS module provides a more convenient way to create secure HTTP servers, but it uses the TLS module under the hood.

TLS with Express

You can also create an HTTPS server with Express:

const express = require('express');
const https = require('https');
const fs = require('fs');
const path = require('path');

// Create Express app
const app = express();

// Define routes
app.get('/', (req, res) => {
  res.send('<h1>Secure Express App</h1><p>This connection is encrypted using TLS.</p>');
});

app.get('/api/data', (req, res) => {
  res.json({
    message: 'This is sensitive data',
    timestamp: new Date()
  });
});

// HTTPS server options
const options = {
  key: fs.readFileSync(path.join(__dirname, 'server-key.pem')),
  cert: fs.readFileSync(path.join(__dirname, 'server-cert.pem'))
};

// Create HTTPS server with Express app
const port = 443;
https.createServer(options, app).listen(port, () => {
  console.log(`Secure Express app running on port ${port}`);
});

Certificate Verification

TLS uses certificates to verify the identity of servers and optionally clients. Here's an example showing how to implement custom certificate verification:

const tls = require('tls');
const fs = require('fs');

// Custom verification function
function validateCertificate(cert) {
  // Basic certificate info
  console.log('Certificate subject:', cert.subject);
  console.log('Certificate issuer:', cert.issuer);
  console.log('Valid from:', cert.valid_from);
  console.log('Valid to:', cert.valid_to);

  // Check certificate validity period
  const now = new Date();
  const validFrom = new Date(cert.valid_from);
  const validTo = new Date(cert.valid_to);

  if (now < validFrom || now > validTo) {
    return { valid: false, reason: 'Certificate is not within its validity period' };
  }

  // Additional checks could include:
  // - Certificate revocation status
  // - Certificate chain validation
  // - Public key strength

  return { valid: true };
}

// Create TLS client with custom validation
const options = {
  ca: [fs.readFileSync('ca-cert.pem')],
  checkServerIdentity: (hostname, cert) => {
    // First check the certificate against our custom rules
    const validationResult = validateCertificate(cert);

    if (!validationResult.valid) {
      return new Error(validationResult.reason);
    }

    // Then verify the hostname matches the certificate
    const certCN = cert.subject.CN;

    if (hostname !== certCN &&
        !cert.subjectaltname ||
        !cert.subjectaltname.includes(hostname)) {
      return new Error(`Certificate name mismatch: ${hostname} !== ${certCN}`);
    }

    // Certificate is valid
    return undefined;
  }
};

// Connect to server with custom verification
const client = tls.connect(8000, 'example.com', options, () => {
  if (client.authorized) {
    console.log('Connection authorized');
    client.write('Secure message');
  } else {
    console.log('Connection not authorized:', client.authorizationError);
  }
});

// Handle connection events
client.on('error', (error) => {
  console.error('TLS error:', error);
});

client.on('end', () => {
  console.log('Connection ended');
});

TLS Session Resumption

Session resumption allows clients to reconnect to a server without performing a full TLS handshake, improving performance:

const tls = require('tls');
const fs = require('fs');
const path = require('path');

// Server options
const serverOptions = {
  key: fs.readFileSync(path.join(__dirname, 'server-key.pem')),
  cert: fs.readFileSync(path.join(__dirname, 'server-cert.pem')),
  // Enable session resumption
  sessionTimeout: 300, // Session timeout in seconds
  ticketKeys: Buffer.from('0123456789abcdef0123456789abcdef'), // 32 bytes for key encryption
};

// Create TLS server
const server = tls.createServer(serverOptions, (socket) => {
  console.log('Client connected');

  // Check if this is a resumed session
  if (socket.isSessionReused()) {
    console.log('Session reused!');
  } else {
    console.log('New session');
  }

  socket.on('data', (data) => {
    console.log('Received:', data.toString());
    socket.write('Hello back!');
  });

  socket.on('end', () => {
    console.log('Client disconnected');
  });
});

server.listen(8443, () => {
  console.log('TLS server listening on port 8443');

  // First client connection
  connectClient(() => {
    // Second client connection - should use session resumption
    connectClient();
  });
});

// Function to create a client with session resumption
let savedSession = null;

function connectClient(callback) {
  const clientOptions = {
    rejectUnauthorized: false, // For self-signed certificates
    session: savedSession // Use saved session if available
  };

  const client = tls.connect(8443, 'localhost', clientOptions, () => {
    console.log('Client connected. Authorized:', client.authorized);
    console.log('Using session resumption:', client.isSessionReused());

    // Save the session for future connections
    savedSession = client.getSession();

    // Send data
    client.write('Hello server!');

    // Close after a short delay
    setTimeout(() => {
      client.end();
      if (callback) setTimeout(callback, 100);
    }, 100);
  });

  client.on('data', (data) => {
    console.log('Client received:', data.toString());
  });

  client.on('error', (err) => {
    console.error('Client error:', err);
  });
}

Server Name Indication (SNI)

SNI allows a server to present different certificates for different hostnames on the same IP address and port:

const tls = require('tls');
const fs = require('fs');
const path = require('path');

// Load different certificates for different domains
const serverOptions = {
  SNICallback: (servername, cb) => {
    console.log(`SNI request for: ${servername}`);

    // Different certificate contexts based on hostname
    if (servername === 'example.com') {
      const context = tls.createSecureContext({
        key: fs.readFileSync(path.join(__dirname, 'example.com-key.pem')),
        cert: fs.readFileSync(path.join(__dirname, 'example.com-cert.pem'))
      });
      cb(null, context);
    }
    else if (servername === 'another.com') {
      const context = tls.createSecureContext({
        key: fs.readFileSync(path.join(__dirname, 'another.com-key.pem')),
        cert: fs.readFileSync(path.join(__dirname, 'another.com-cert.pem'))
      });
      cb(null, context);
    }
    else {
      // Default certificate
      const context = tls.createSecureContext({
        key: fs.readFileSync(path.join(__dirname, 'default-key.pem')),
        cert: fs.readFileSync(path.join(__dirname, 'default-cert.pem'))
      });
      cb(null, context);
    }
  },
  // Default keys and certificates (used as a fallback)
  key: fs.readFileSync(path.join(__dirname, 'default-key.pem')),
  cert: fs.readFileSync(path.join(__dirname, 'default-cert.pem'))
};

// Create server
const server = tls.createServer(serverOptions, (socket) => {
  socket.write(`Hello, you connected to ${socket.servername || 'unknown'}!\n`);
  socket.end();
});

server.listen(8443, () => {
  console.log('TLS SNI server running on port 8443');
});

Advanced Certificate Management

Proper certificate management is crucial for secure TLS communications. Here are some advanced techniques:

1. Certificate Chain and Multiple CAs

const tls = require('tls');
const fs = require('fs');
const path = require('path');

// Load multiple CA certificates
const caCerts = [
  fs.readFileSync(path.join(__dirname, 'ca1-cert.pem')),
  fs.readFileSync(path.join(__dirname, 'ca2-cert.pem')),
  fs.readFileSync(path.join(__dirname, 'intermediate-cert.pem'))
];

// Server with multiple CA certificates
const serverOptions = {
  key: fs.readFileSync(path.join(__dirname, 'server-key.pem')),
  cert: fs.readFileSync(path.join(__dirname, 'server-cert.pem')),
  ca: caCerts,  // Array of CA certificates
  requestCert: true,
  rejectUnauthorized: true
};

const server = tls.createServer(serverOptions, (socket) => {
  console.log('Client connected:', socket.authorized ? 'Authorized' : 'Unauthorized');

  // Get peer certificate
  const cert = socket.getPeerCertificate();
  console.log('Client certificate subject:', cert.subject);
  console.log('Issuer:', cert.issuer.CN);

  socket.write('Welcome to the secure server!\n');
  socket.end();
});

server.listen(8000, () => {
  console.log('TLS server running on port 8000');
});

2. Certificate Revocation with CRL

const tls = require('tls');
const fs = require('fs');
const crypto = require('crypto');

// Load CRL (Certificate Revocation List)
const crl = fs.readFileSync('revoked-certs.pem');

// Parse CRL to check against
const checkRevocation = (cert) => {
  // In a real application, you would parse the CRL and check
  // if the certificate's serial number is in the revocation list

  // For demonstration, we'll just check against a known revoked serial
  const revokedSerials = [
    '0123456789ABCDEF', // Example revoked serial
    'FEDCBA9876543210'
  ];

  const certInfo = crypto.certificateVerify(
    cert.raw,
    'sha256',
    Buffer.from(''),
    Buffer.from('')
  );

  return !revokedSerials.includes(certInfo.serialNumber.toString('hex').toUpperCase());
};

const server = tls.createServer({
  key: fs.readFileSync('server-key.pem'),
  cert: fs.readFileSync('server-cert.pem'),
  requestCert: true,
  rejectUnauthorized: true,

  // Custom certificate validation
  checkServerIdentity: (host, cert) => {
    if (!checkRevocation(cert)) {
      return new Error('Certificate has been revoked');
    }
    return undefined; // No error means certificate is valid
  }
}, (socket) => {
  // Handle connection
  console.log('Client connected:', socket.authorized ? 'Authorized' : 'Unauthorized');
  socket.end('Hello secure world!\n');
});

server.listen(8000);

3. Automatic Certificate Management with Let's Encrypt

const tls = require('tls');
const https = require('https');
const fs = require('fs');
const path = require('path');
const { execSync } = require('child_process');

class TLSCertManager {
  constructor(domain, email) {
    this.domain = domain;
    this.email = email;
    this.certDir = path.join(__dirname, 'certs', domain);
    this.ensureCertDir();
  }

  ensureCertDir() {
    if (!fs.existsSync(this.certDir)) {
      fs.mkdirSync(this.certDir, { recursive: true });
    }
  }

  async getCertificates() {
    const keyPath = path.join(this.certDir, 'privkey.pem');
    const certPath = path.join(this.certDir, 'cert.pem');
    const chainPath = path.join(this.certDir, 'chain.pem');

    // Check if certificates exist and are valid
    if (this.certsValid(keyPath, certPath, chainPath)) {
      return {
        key: fs.readFileSync(keyPath),
        cert: fs.readFileSync(certPath),
        ca: fs.readFileSync(chainPath)
      };
    }

    // Use certbot to obtain new certificates
    return await this.obtainCertificates();
  }

  certsValid(keyPath, certPath, chainPath) {
    try {
      if (!fs.existsSync(keyPath) || !fs.existsSync(certPath) || !fs.existsSync(chainPath)) {
        return false;
      }

      // Check if certificate is valid for at least 7 more days
      const cert = fs.readFileSync(certPath);
      const notAfter = cert.toString().match(/Not After : (.*?)\n/)[1];
      const expiryDate = new Date(notAfter);
      const now = new Date();

      return (expiryDate - now) > 7 * 24 * 60 * 60 * 1000; // 7 days in ms
    } catch (err) {
      console.error('Error checking certificate validity:', err);
      return false;
    }
  }

  async obtainCertificates() {
    try {
      // This is a simplified example - in production, use a proper ACME client
      console.log('Obtaining new certificates from Let\'s Encrypt...');

      // In a real application, you would use an ACME client like 'greenlock' or 'acme'
      // This is just a placeholder to illustrate the concept
      execSync(`certbot certonly --standalone -d ${this.domain} --email ${this.email} --agree-tos --non-interactive`);

      // Copy certificates to our certs directory
      const certs = {
        key: fs.readFileSync(`/etc/letsencrypt/live/${this.domain}/privkey.pem`),
        cert: fs.readFileSync(`/etc/letsencrypt/live/${this.domain}/cert.pem`),
        ca: fs.readFileSync(`/etc/letsencrypt/live/${this.domain}/chain.pem`)
      };

      // Save certificates for future use
      fs.writeFileSync(path.join(this.certDir, 'privkey.pem'), certs.key);
      fs.writeFileSync(path.join(this.certDir, 'cert.pem'), certs.cert);
      fs.writeFileSync(path.join(this.certDir, 'chain.pem'), certs.ca);

      return certs;
    } catch (err) {
      console.error('Failed to obtain certificates:', err);
      throw err;
    }
  }
}

// Usage example
async function createSecureServer() {
  const certManager = new TLSCertManager('example.com', 'admin@example.com');

  try {
    const certs = await certManager.getCertificates();

    const server = https.createServer({
      key: certs.key,
      cert: certs.cert,
      ca: certs.ca,
      requestCert: true,
      rejectUnauthorized: true
    }, (req, res) => {
      res.writeHead(200);
      res.end('Hello, secure world!\n');
    });

    server.listen(443, () => {
      console.log('HTTPS server running on port 443');
    });

    // Schedule certificate renewal check (e.g., daily)
    setInterval(async () => {
      try {
        await certManager.getCertificates();
      } catch (err) {
        console.error('Certificate renewal check failed:', err);
      }
    }, 24 * 60 * 60 * 1000); // Check daily

  } catch (err) {
    console.error('Failed to start secure server:', err);
    process.exit(1);
  }
}

createSecureServer();

Note: The Let's Encrypt example is simplified. In production, use a well-maintained ACME client library and follow Let's Encrypt's rate limits and best practices.

Security Best Practices

When using TLS in production applications, consider these security best practices:

1. Use Strong TLS Versions

const options = {
  // Disable older TLS versions
  minVersion: 'TLSv1.2',

  // Explicitly disallow TLS 1.0 and 1.1
  secureOptions: crypto.constants.SSL_OP_NO_TLSv1 |
                 crypto.constants.SSL_OP_NO_TLSv1_1
};

2. Configure Strong Cipher Suites

const options = {
  // Prioritize modern, secure cipher suites
  ciphers: [
    'TLS_AES_256_GCM_SHA384',
    'TLS_CHACHA20_POLY1305_SHA256',
    'TLS_AES_128_GCM_SHA256',
    'ECDHE-RSA-AES256-GCM-SHA384',
    'ECDHE-RSA-AES128-GCM-SHA256'
  ].join(':')
};

3. Use Perfect Forward Secrecy

// Cipher suites with ECDHE (Elliptic Curve Diffie-Hellman Ephemeral) support PFS
const options = {
  ciphers: 'ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384'
};

4. Implement OCSP Stapling (Online Certificate Status Protocol)

const tls = require('tls');
const https = require('https');
const fs = require('fs');
const path = require('path');

// Server with OCSP stapling
const serverOptions = {
  key: fs.readFileSync(path.join(__dirname, 'server-key.pem')),
  cert: fs.readFileSync(path.join(__dirname, 'server-cert.pem')),
  ca: fs.readFileSync(path.join(__dirname, 'ca-cert.pem')),

  // Enable OCSP stapling
  requestOCSP: true,

  // OCSP response cache timeout (in milliseconds)
  ocspCache: new tls.OCSPCache({
    max: 1000,  // Maximum number of cached responses
    maxAge: 60 * 60 * 1000  // Cache for 1 hour
  })
};

// Create HTTPS server with OCSP stapling
const server = https.createServer(serverOptions, (req, res) => {
  res.writeHead(200);
  res.end('Hello with OCSP stapling!\n');
});

// Handle OCSP request errors
server.on('OCSPRequest', (cert, issuer, callback) => {
  if (!cert || !issuer) {
    return callback(new Error('No certificate or issuer provided'));
  }

  // Get OCSP URL from certificate
  const ocspUrl = tls.getOCSPURL(cert);
  if (!ocspUrl) {
    return callback(new Error('No OCSP URL in certificate'));
  }

  console.log('OCSP request for:', cert.subject.CN);

  // In a real application, you would make an OCSP request here
  // and return the response via the callback

  // For demonstration, we'll just return a dummy response
  const ocspResponse = Buffer.from('OCSP response would go here');
  callback(null, ocspResponse);
});

server.listen(443, () => {
  console.log('HTTPS server with OCSP stapling running on port 443');
});

// Client that verifies OCSP stapling
const clientOptions = {
  host: 'example.com',
  port: 443,
  rejectUnauthorized: true,
  requestOCSP: true  // Request OCSP stapling from server
};

const req = https.request(clientOptions, (res) => {
  console.log('Response status code:', res.statusCode);

  // Get the OCSP response from the server
  const ocspResponse = res.socket.getOCSPResponse();
  if (ocspResponse) {
    console.log('Received OCSP response');
    // Verify the OCSP response here
  } else {
    console.log('No OCSP response received');
  }

  res.on('data', (chunk) => {
    console.log('Received data:', chunk.toString());
  });
});

req.on('error', (err) => {
  console.error('Request error:', err);
});

req.end();

5. ALPN and SNI Support

Application-Layer Protocol Negotiation (ALPN) and Server Name Indication (SNI) are important TLS extensions that enable protocol negotiation and virtual hosting:

const tls = require('tls');
const http2 = require('http2');
const https = require('https');
const fs = require('fs');
const path = require('path');

// Server with ALPN and SNI support
const serverOptions = {
  // ALPN protocols in order of preference
  ALPNProtocols: ['h2', 'http/1.1'],

  // SNI callback for multiple domains
  SNICallback: (servername, cb) => {
    console.log('SNI request for:', servername);

    try {
      let context;

      // Create different contexts for different domains
      if (servername === 'example.com') {
        context = tls.createSecureContext({
          key: fs.readFileSync(path.join(__dirname, 'example.com-key.pem')),
          cert: fs.readFileSync(path.join(__dirname, 'example.com-cert.pem')),
          // Enable OCSP stapling for this domain
          requestOCSP: true,
          // Custom cipher suites for this domain
          ciphers: [
            'TLS_AES_256_GCM_SHA384',
            'TLS_CHACHA20_POLY1305_SHA256',
            'TLS_AES_128_GCM_SHA256'
          ].join(':')
        });
      } else {
        // Default context for other domains
        context = tls.createSecureContext({
          key: fs.readFileSync(path.join(__dirname, 'default-key.pem')),
          cert: fs.readFileSync(path.join(__dirname, 'default-cert.pem')),
          // Less strict ciphers for legacy clients
          ciphers: [
            'TLS_AES_256_GCM_SHA384',
            'TLS_CHACHA20_POLY1305_SHA256',
            'TLS_AES_128_GCM_SHA256',
            'ECDHE-RSA-AES256-GCM-SHA384',
            'ECDHE-RSA-AES128-GCM-SHA256'
          ].join(':')
        });
      }

      // Set ALPN protocols for this context
      context.setALPNProtocols(['h2', 'http/1.1']);

      // Return the created context
      if (cb) {
        cb(null, context);
      } else {
        return context;
      }
    } catch (err) {
      console.error('SNI callback error:', err);
      if (cb) {
        cb(err);
      } else {
        throw err;
      }
    }
  },

  // Default key and cert (used if SNI is not supported by client)
  key: fs.readFileSync(path.join(__dirname, 'default-key.pem')),
  cert: fs.readFileSync(path.join(__dirname, 'default-cert.pem'))
};

// Create HTTP/2 server with ALPN and SNI
const http2Server = http2.createSecureServer(serverOptions, (req, res) => {
  const protocol = req.socket.alpnProtocol;
  res.writeHead(200);
  res.end(`Hello from ${req.socket.servername} using ${protocol}\n`);
});

http2Server.on('error', (err) => {
  console.error('HTTP/2 server error:', err);
});

http2Server.on('stream', (stream, headers) => {
  const protocol = stream.session.alpnProtocol;
  const hostname = stream.session.servername || 'unknown';

  stream.respond({
    'content-type': 'text/plain; charset=utf-8',
    ':status': 200
  });

  stream.end(`HTTP/2 stream from ${hostname} using ${protocol}\n`);
});

// Create HTTPS server with ALPN and SNI
const httpsServer = https.createServer(serverOptions, (req, res) => {
  const protocol = req.socket.alpnProtocol;
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end(`Hello from ${req.socket.servername} using ${protocol || 'HTTP/1.1'}\n`);
});

// Handle upgrade to HTTP/2
httpsServer.on('upgrade', (req, socket, head) => {
  const protocol = req.socket.alpnProtocol;
  if (protocol === 'h2') {
    http2Server.emit('connection', socket);
  } else {
    socket.destroy();
  }
});

// Start servers
const PORT = 443;

httpsServer.listen(PORT, () => {
  console.log(`HTTPS server running on port ${PORT}`);
});

// Client example
function makeRequest(hostname, port = 443) {
  const options = {
    hostname,
    port,
    path: '/',
    method: 'GET',
    // Enable ALPN
    ALPNProtocols: ['h2', 'http/1.1'],
    // Set SNI
    servername: hostname,
    // Verify certificate
    rejectUnauthorized: false, // For testing with self-signed certs
    // Custom check for server identity
    checkServerIdentity: (host, cert) => {
      // Implement custom certificate validation
      return undefined; // No error means success
    }
  };

  const req = https.request(options, (res) => {
    console.log(`Status: ${res.statusCode}`);
    console.log('ALPN Protocol:', res.socket.alpnProtocol);
    console.log('Negotiated Protocol:', res.socket.getProtocol());

    let data = '';
    res.on('data', (chunk) => {
      data += chunk;
    });

    res.on('end', () => {
      console.log('Response:', data.trim());
    });
  });

  req.on('error', (err) => {
    console.error('Request error:', err);
  });

  req.end();
}

// Example usage
// makeRequest('example.com');
// makeRequest('another-domain.com');

6. Use HTTP Strict Transport Security (HSTS)

// In an Express application
app.use((req, res, next) => {
  res.setHeader('Strict-Transport-Security', 'max-age=31536000; includeSubDomains; preload');
  next();
});